1. Field of the Invention
The present invention relates to a wireless transmitter for use in a communication system that uses subcarriers, such as OFDM (Orthogonal Frequency Division Multiplex).
2. Prior Art
Generally speaking, in a modulated signal subjected to amplitude modulation, in particular, a modulated signal subjected to multi-level modulation, such as QAM (Quadrature Amplitude Modulation), linear operation is required in a high-frequency power amplifier to transmit power to an antenna. Hence, class A or class AB has been used as the operation class of the high-frequency power amplifier.
However, as broadband communication is promoted, a communication system that uses subcarriers, such as OFDM (Orthogonal Frequency Division Multiplex), has begun to be used, and the conventional class-A or class-AB high-frequency power amplifier cannot be expected to attain high efficiency. In other words, in OFDM, subcarriers are overlapping, whereby high power is generated instantaneously at random. That is to say, the ratio PAPR (Peak to Average Power Ratio) of the instantaneous maximum power to average power is high. Hence, in order that the instantaneous maximum power can also be amplified linearly, it is necessary to hold high DC power at all times. The power efficiency of the class-A operation is only 50% at the maximum; in particular, in the case of OFDM, since PAPR is high, the power efficiency is only about 10%.
For this reason, in the case of a portable wireless transmitter wherein batteries are used as a power supply, its operable time is short, thereby causing a problem in practical use.
In order to solve this kind of problem, the conventional EER (Envelope Elimination and Restoration) method has been proposed, which is known as Kahn's method (for example, see Patent document 1).
FIG. 7 is a block diagram showing the schematic configuration of the EER method. In FIG. 7, a modulated signal, such as a QAM signal, output from an OFDM signal generating means 100 serving as a modulated signal generating means, is divided into two branches. In one of the branches, the QAM signal is up-converted using an orthogonal modulation circuit 106 and input as a high-frequency modulated wave to the high-frequency input terminal of a high-frequency power amplifier 110 formed of a saturation-type amplifier. In addition, in the other branch, the QAM signal is converted into an amplitude component using an amplitude extracting means 101.
A DC current required to drive the high-frequency power amplifier 110 is supplied to an amplitude amplifying means 102 from a DC power supply having a power voltage Vdd1. Hence, the amplitude amplifying means 102 amplifies the amplitude component by a preset gain and supplies the component to the power voltage terminal of the high-frequency power amplifier 110.
When the power supply voltage of the saturation-type amplifier serving as the high-frequency power amplifier 110 is controlled in proportion to the amplitude component of the high-frequency modulated wave serving as an input signal as described above, the high-frequency modulated wave including the original amplitude component is restored and output from the saturation-type amplifier.
With this configuration, even if the amplitude component of the modulated signal changes, the high-frequency power amplifier formed of the saturation-type amplifiers can be operated in a highly efficient saturated state, and high efficiency can be attained.
In the case that the high-frequency power amplifier is formed of a field effect transistor, for example, the saturation-type amplifier is a class-F amplifier wherein harmonics are controlled so that the waveform of the drain voltage becomes rectangular, or a class-E or class-D amplifier wherein load conditions are optimized so that the waveform of the drain voltage and the waveform of the drain current do not overlap each other. The saturation-type amplifier is characterized in that its power consumption can be suppressed since the period in which the drain voltage and the drain current are generated simultaneously is made as short as possible.
When a power current of 200 mA and a power voltage of 3 V, for example, are supplied to an amplifier, a DC power of 600 mW is obtained. However, in the case of the saturation-type amplifier serving as the high-frequency power amplifier 110, when the field effect transistor is OFF, no current flows but only the power supply voltage is applied. Hence, the DC power consumption is zero. On the other hand, when the field effect transistor is ON, a current of 200 mA flows. However, since the field effect transistor conducts completely, the voltage VDS between the drain and the source can be assumed to be about 0.3 V at most. In this case, a DC power of 0.3 V×0.2 A=0.06 W, that is, 60 mW, is consumed inside the field effect transistor. Hence, the power efficiency reaches a very high value of (600−60)/600=90%. This effect is significant in comparison with the fact that the power efficiency of the class-A amplifier is only 50% at the maximum.
Furthermore, generally speaking, in a transmitter employing the EER method, unless the amplitude component and the phase component at the output terminal of the high-frequency power amplifier are not the accurate reproduction of the amplitude component and the phase component of the original modulated signal, the original modulated signal cannot be reproduced. Since the original modulated signal has been frequency-modulated, the high-frequency modulated wave cannot be reproduced accurately.
Eventually, the errors of the amplitude component and the phase component are revealed by the spectrum distortion or deterioration in the accuracy of modulation in the high-frequency modulated wave to be output.
Hence, in the EER method, a modulated signal, subjected to inverse correction wherein the error functions of the amplitude component and the phase component are obtained beforehand and the inverse functions of the error functions are multiplied, is required to be output from the OFDM signal generating means 100.
In addition, as the modulated signal for obtaining the error functions, a modulated signal, the data rate of which is lower than that of the OFDM modulated signal, is used to reduce the arithmetic processing load at the time when the inverse functions are obtained from the error functions.
Hence, a delay time occurs between the amplitude component and the phase component of the OFDM modulated signal that uses a high-speed data rate, thereby causing spectrum distortion or deterioration in the accuracy of modulation in the high-frequency modulated wave.
Therefore, generally speaking, in a transmitter employing the EER method, the original modulated signal is reproduced by providing a means for correcting the phase delay time of the phase component of the modulated signal and by optimizing the delay time.
Patent document 1: U.S. Pat. No. 6,256,482B1
Patent document 2: U.S. Pat. No. 5,251,330A1
However, in the conventional transmitters, the high-frequency power amplifier comprises multiple stages of amplifiers being connected in series, and the amplitude component being output from an amplitude amplifying means is branched and supplied as the power voltages of two or more amplifiers.
In this configuration, when such a high-speed data rate of 54 Mbps at the maximum as in the wireless LAN IEEE802.11a/g Standard is used as the data rate of the modulated signal, the amount of the delay time between the input and output of the final-stage amplifier at the time when the phase component passes through the final-stage amplifier cannot be ignored in comparison with the data rate.
In other words, in the case that an amplitude component is divided into two branches, for example, and supplied to the power supply terminals of the final-stage amplifier and the other amplifier, the delay time between the phase component and the amplitude component of the final-stage amplifier can be corrected using a means for correcting the phase delay time of the phase component of the modulated signal, and the phase component and the amplitude component can be synthesized. However, in the other amplifier, errors occur in the phase component and the amplitude component by the amount of the delay time between the input and output of the final-stage amplifier at the time when the phase component passes through the final-stage amplifier, and optimal synthesis cannot be attained. In other words, the modulated wave cannot be reproduced accurately at the output terminal of the saturation-type amplifier but is distorted. As a result, there is a problem of causing spectrum distortion or deterioration in the accuracy of modulation in the high-frequency modulated wave.